Apparatus and method for converting image data

ABSTRACT

An image data converting apparatus comprising an MPEG2-image data decoding section  19 , a scan-converting section  20 , an MPEG4-image encoding section  21 , and a picture-type determining section  18 . The MPEG2-image data decoding section  19  decodes input MPEG2-image compressed data in both the vertical direction and the horizontal direction by using only lower, fourth-order coefficients. The scan-converting section  20  converts interlaced-scan pixel data to sequential-scan pixel data. The MPEG4-image encoding section  21  generates MPEG4-image compressed data from the sequential-scan pixel signals. The sections  19, 20  and  21  are connected in series. The picture-type determining section  18  is connected to the input of the MPEG2-image data decoding section  19 . The section  18  determines the picture type of each frame data contained in the interlaced-scan MPEG4-picture compressed data, outputs only the frame data about I/P pictures, discards the frame data about B pictures, thereby converting the frame rate.

BACKGROUND OF THE INVENTION

The present invention relates to an apparatus and method for convertingimage data. The apparatus and the method are suitable in receivingcompressed image data (i.e., bit streams) through network media (e.g.,satellite broadcast, cable television and the Internet). The image datamay be one that has been compressed by orthogonal transformation (e.g.,discrete cosine transform) and motion compensation, as in an MPEG(Moving Picture image coding Experts Group) system. The apparatus andthe method are suitable, too, in processing image data recorded onstorage media such as optical disks and magnetic disks.

In recent years, more and more apparatuses complying with the MPEGsystem are used in data-distributing facilities such as broadcaststations and data-receiving sites such as households. These apparatusesperform orthogonal transformation (e.g., discrete cosine transform) andmotion compensation on digital image data that has redundancy, therebycompressing the image data. The image data can be transmitted and storedwith a higher efficiency than in the case it is not so compressed atall.

In particular, MPEG2 (ISO/IEC 13818-2) is defined as a general-purposesystem for encoding image data. This will be applied widely toprofessional use and consumer use, as a standard system that processesvarious types of image data, including interlaced-scan image data,sequential-scan image data, standard-resolution image data, andhigh-definition image data. If the MPEG2 data-compressing system isutilized, a high compression ratio and high-quality images will beachieved by allocating a bit rate of 4 to 8 Mbps to interlaced-scanimages of standard resolution, each having 720×480 pixels. Also, a highcompression ratio and high-quality images will be achieved by allocatinga bit rate of 18 to 22 Mbps to interlaced-scan images of highresolution, each having 1920×1088 pixels.

This data-compressing system will be used to transmit image data in thedigital broadcasting that will be put to general use. The system isdesigned to transmit not only standard-resolution image data but alsohigh-resolution image data. The receiver needs to decode the two typesof image data. To provide an inexpensive receiver that can decode bothtypes of image data, it is required that some data be extracted from thehigh-resolution image data, while minimizing the inevitabledeterioration of image quality. This requirement should be fulfilled notonly in transmission media such as digital broadcasting, but also instorage media such as optical disks and flash memories.

To fulfill the requirement, the inventor hereof has proposed an imagedata decoding apparatus of the type shown in FIG. 1. As shown in FIG. 1,the apparatus comprises an code buffer 101, a data-compressing/analyzingsection 102, a variable-length decoding section 103, an inversequantization section 104, motion-compensating sections 108 and 109,video memory 110 and an adder 107. These components are basicallyidentical in function to those incorporated in the ordinary MPEGencoding apparatuses.

In the apparatus of FIG. 1, the compressed image data input is suppliedvia the code buffer 101 to the data-compressing/analyzing section 102.The data-compressing/analyzing section 102 analyzes the compressed imagedata, thereby obtaining data that will be used to expand the image data.The data thus obtained is supplied to the variable-length decodingsection 103, together with the compressed image data. Thevariable-length decoding section 103 performs variable-length encoding,i.e., a process reverse to the variable-length encoding that has beenperformed to generate the compressed image data. In the decoding section103, however, only coefficients may be decoded and no other process maybe carried out until the EOB (End of Block) is detected. Thesecoefficients are required in a compression inverse discrete-cosinetransform (4×4) section 105 or a compression inverse discrete-cosinetransform (field separation) section 106, in accordance with whether themacro block is of field DCT mode or fame DCT mode. FIG. 2A and FIG. 2Bshow two operating principles the variable-length decoding section 103assume to decode MPEG2-image compressed data (bit stream) that has beengenerated by zigzag scanning. More precisely, FIG. 2A depicts theoperating principle that the decoding section 103 assumes to decode thecompressed data in field DCT mode, and FIG. 2B illustrates the operatingprinciple that the decoding section 103 assumes to decode the compresseddata in frame DCT mode. FIG. 3A and FIG. 3B show two operatingprinciples the variable-length decoding section 103 assume to decodeMPEG2-image compressed data (bit stream) that has been generated byalternate scanning. To be more specific, FIG. 3A depicts the operatingprinciple that the decoding section 103 assumes to decode the compresseddata in field DCT mode, and FIG. 3B illustrates the operating principlethat the decoding section 103 assumes to decode the compressed data inframe DCT mode. The numbers in FIGS. 2A, 2B, 3A and 3B indicate theorder in which the data items have been generated by scanning. The datadecoded by the variable-length decoding section 103 is supplied to theinverse quantization section 104. The inverse quantization section 104performs inverse quantization on the input data. The data generated bythe section 104 is supplied to the compression inverse discrete-cosinetransform (4×4) section 105 or the compression inverse discrete-cosinetransform (field separation) section 106. The section 105 or 106performs inverse discrete-cosine transform on the input data.

The inverse quantization section 104 performs inverse quantization,generating a discrete-cosine transform coefficient. The discrete-cosinetransform coefficient is supplied to either the discrete-cosinetransform (4×4) section 105 or the discrete-cosine transform (fieldseparation) section 106, in accordance with whether the macro block isof the field DCT mode or the frame DCT mode. The section 105 or thesection 106 performs inverse discrete-cosine transform on the inputdata.

The macro block may be an intra macro block. In this case, the datasubjected to compression inverse discrete-cosine transform is stored viathe adder 107 into a video memory 110, without being further processedat all. If the macro block is an inter macro block, the data is suppliedto the motion-compensating section 108 or the motion-compensatingsection 109, in accordance with whether the motion-compensating mode isa field-predicting mode or a frame-predicting mode. The section 108 or109 effects interpolation which achieves ¼ pixel precision in both thehorizontal direction and the vertical direction, by using the referencedata stored in the video memory 110, thereby generating predicted pixeldata. The predicted pixel data is supplied to the adder 107, togetherwith the pixel data subjected to the inverse discrete-cosine transform.The adder 107 adds the predicted pixel data and the pixel data,generating a pixel value. The pixel value is stored into the videomemory 110. As shown in FIGS. 4A and 4B, the lower-layer pixel valuestored in the video memory 110 contains a phase difference between thefirst and second fields, with respect to the upper-layer pixel value.The circles shown in FIGS. 4A and 4B indicate the pixels.

The image data-decoding apparatus has a frame-converting section 111.The frame-converting section 111 converts the pixel value, which isstored in the video memory 110, to an image signal that represents animage of such a size as can be displayed by a display (not shown). Theimage signal output from the frame-converting section 111 is the decodedimage signal that is output from the image decoding apparatus of FIG. 1.

The operating principles of the compression inverse discrete-cosinetransform (4×4) section 105 and compression inverse discrete-cosinetransform (field separation) section 106 will be described.

The compression inverse discrete-cosine transform (4×4) section 105extracts the lower, fourth-order coefficients included in eighth-orderdiscrete cosine transform coefficients, for both the horizontalcomponent and the vertical component. Then, the section 105 performsfourth-order inverse discrete cosine transform on the fourth-ordercoefficients extracted.

On the other hand, the compression inverse discrete-cosine transformsection 106 carries out the process that will be described below.

FIG. 5 illustrates the sequence of operations that the compressioninverse discrete-cosine transform section 106 performs.

As shown in FIG. 5, the compression inverse discrete-cosine transform(field separation) section 106 first performs 8×8 inverse discretecosine transform (IDCT) on the discrete cosine transform coefficients y₁to y₈ contained in the compressed image data (bit stream) that is theinput data. Data items x₁ to x₈, or decoded data items, are therebygenerated. Then, the section 106 separates the data items x₁ to x₈ intotwo first field data and second field data. The first field dataconsists of the data items x₁, x₃, x₅ and x₇. The second field dataconsists of data items x₂, x₄, x6 and x₈. Next, the section 106 performs4×4 discrete cosine transform (DCT) on the first field data, generatingdiscrete cosine transform coefficients z₁, z₃, z₅ and z₇, and on thesecond field data, generating discrete cosine coefficients z₂, z₄, z6and z₈. Further, the section 106 performs 2×2 inverse discrete cosinetransform on only the lower ones of each field data. Thus, compressedpixel values x′₁ an x′₃ are obtained for the first field data, andcompressed pixel values x′₂ an x′₄ are obtained for the second fielddata. Then, the pixel values are subjected to frame synthesis,generating output values x′₁, x′₂, x′₃ and x′₄. In practice, the pixelvalues x′₁, x′₂, x′₃ and x′₄ are obtained by effecting a matrixalgebraic operation equivalent to this sequence of operations, on thediscrete cosine transform coefficients y₁ to y₈. The matrix [FS¹]obtained by calculation using addition theorem is given as follows:equation 1: $\begin{matrix}{\left\lbrack {FS}^{I} \right\rbrack = {\frac{1}{\sqrt{2}}\begin{bmatrix}A & B & D & {- E} & F & G & H & I \\A & {- C} & {- D} & E & {- F} & {- G} & {- H} & {- J} \\A & C & {- D} & {- E} & {- F} & G & {- H} & J \\A & {- B} & D & E & F & {- G} & H & {- I}\end{bmatrix}}} & (1)\end{matrix}$

A to J in the equation (1) are as follows: equation  2:$\quad{A = \frac{1}{\sqrt{2}}}$$\quad{B = \frac{{\cos\left( \frac{\pi}{16} \right)} + {\cos\left( \frac{3\pi}{16} \right)} + {3\quad{\cos\left( \frac{5\pi}{16} \right)}} - {\cos\left( \frac{7\pi}{16} \right)}}{4}}$$\quad{D = \frac{{\cos\left( \frac{\pi}{16} \right)} - {3{\cos\left( \frac{3\pi}{16} \right)}} - {\cos\left( \frac{5\pi}{16} \right)} - {\cos\left( \frac{7\pi}{16} \right)}}{4}}$$\quad{D = \frac{1}{4}}$$\quad{E = \frac{{\cos\left( \frac{\pi}{16} \right)} - {\cos\left( \frac{3\pi}{16} \right)} - {\cos\left( \frac{5\pi}{16} \right)} - {\cos\left( \frac{7\pi}{16} \right)}}{4}}$$\quad{F = \frac{{\cos\left( \frac{\pi}{8} \right)} + {\cos\left( \frac{3\pi}{8} \right)}}{2}}$$\quad{G = \frac{{\cos\left( \frac{\pi}{16} \right)} - {\cos\left( \frac{3\pi}{16} \right)} + {\cos\left( \frac{5\pi}{16} \right)} + {\cos\left( \frac{7\pi}{16} \right)}}{4}}$$\quad{H = {\frac{1}{4} + \frac{1}{2\sqrt{2}}}}$$\quad{I = \frac{{\cos\left( \frac{\pi}{16} \right)} - {\cos\left( \frac{3\pi}{16} \right)} + {3\quad{\cos\left( \frac{5\pi}{16} \right)}} + {\cos\left( \frac{7\pi}{16} \right)}}{4}}$$\quad{J = \frac{{\cos\left( \frac{\pi}{16} \right)} + {3{\cos\left( \frac{3\pi}{16} \right)}} - {\cos\left( \frac{5\pi}{16} \right)} - {\cos\left( \frac{7\pi}{16} \right)}}{4}}$

The operations of the compression inverse discrete-cosine transform(4×4) section 105 and compression inverse discrete-cosine transform(field separation) section 106 can be carried out by applying a fastalgorithm. An example of a fast algorithm is the Wang algorithm (seeZhong de Wang, “Fast Algorithms for the Discrete W Transform and for theDiscrete Fourier Transform,” IEEE Tr. ASSP-32, No. 4, pp. 803-816,August 1984).

The matrix representing the compression discrete cosine transform (4×4)that the section 105 performs can be decomposed as shown below, byapplying the Wang algorithm: $\begin{matrix}\begin{matrix}{{{equation}\quad 3{\text{:}\quad\left\lbrack C_{4}^{II} \right\rbrack}^{- 1}} = {\begin{bmatrix}1 & 0 & 0 & 1 \\0 & 1 & 1 & 0 \\0 & 1 & {- 1} & 0 \\1 & 0 & 0 & {- 1}\end{bmatrix} \cdot {\quad{\begin{bmatrix}\left\lbrack C_{2}^{III} \right\rbrack & \quad \\\quad & \left\lbrack \overset{\_}{C_{2}^{III}} \right\rbrack\end{bmatrix} \cdot \begin{bmatrix}1000 \\0010 \\0001 \\0100\end{bmatrix}}}}} & \quad\end{matrix} & (2)\end{matrix}$

The matrix [C¹¹¹ ₂] in the equation (2) is expressed as follows:$\begin{matrix}{{{equation}{\quad\quad}4{\text{:}\quad\left\lbrack C_{2}^{III} \right\rbrack}} = {\left\lbrack C_{2}^{II} \right\rbrack^{T} = {\frac{1}{\sqrt{2}}\begin{bmatrix}1 & 1 \\1 & {- 1}\end{bmatrix}}}} & \quad \\{{{{equation}\quad 5\text{:}}\left\lbrack \overset{\_}{C_{2}^{III}} \right\rbrack} = {\left\lbrack \quad\begin{matrix}{- C_{\frac{1}{8}}} & C_{\frac{3}{8}} \\C_{\frac{3}{8}} & C_{\frac{1}{8}}\end{matrix} \right\rbrack = {\left\lbrack \quad\begin{matrix}1 & 0 & {- 1} \\0 & 1 & 1\end{matrix} \right\rbrack \cdot {\quad{\begin{bmatrix}{{- C_{\frac{1}{8}}} + C_{\frac{3}{8}}} & 0 & 0 \\0 & {C_{\frac{1}{8}} + C_{\frac{3}{8}}} & 0 \\0 & 0 & C_{\frac{3}{8}}\end{bmatrix} \cdot \begin{bmatrix}1 & 0 \\0 & 1 \\1 & {- 1}\end{bmatrix}}}}}} & \quad\end{matrix}$

In these equations, C_(r)=cos (rδ).

FIG. 6 illustrates how the compression inverse discrete-cosine transform(4×4) section 105 performs a 4×4 compression inverse discrete-cosinetransform by using the above-mentioned Wang algorithm.

As shown in FIG. 6, an adder 121 adds coefficients F(0) and F(2) (i.e.,two of lower, fourth-order coefficients F(0) to F(3)), and an adder 122adds an inverted coefficient F(2) to coefficient F(0), therebyperforming subtraction. A multiplier 123 multiplies the output of theadder 121 by a coefficient A (=½). The product obtained by themultiplier 123 is supplied to adders 133 and 134. Meanwhile, amultiplier 124 multiplies the output of the adder 122 by the coefficientA. The product obtained by the multiplier 124 is supplied to adders 131and 132.

An adder 125 adds an inverted coefficient F(1) to the coefficient F(3),thereby effecting subtraction. A multiplier 128 multiplies the output ofthe adder 125 by a coefficient D (=C3/8). The product obtained by themultiplier 128 is supplied to an adder 130, and is inverted and thensupplied to an adder 129.

A multiplier 126 multiplies the coefficient F(3) by a coefficient B(=C_(1/8)+C_(3/8)). The product obtained by the multiplier 126 issupplied to the adder 129. A multiplier 127 multiplies the coefficientF(1) by a coefficient C (=C_(1/8)+C_(3/8)). The product obtained by themultiplier 127 is supplied to the adder 130.

The adder 129 adds the inverted output of the adder 128 to the output ofthe multiplier 126, thus performing subtraction. The adder 130 adds theoutputs of the multipliers 127 and 128. The output of the adder 129 issupplied to the adder 131, and is inverted and then supplied to theadder 132. Meanwhile, the output of the adder 130 is supplied to theadder 133, and is inverted and then supplied to the adder 134.

The adder 131 adds the output of the multiplier 124 and the output ofthe adder 129. The adder 132 adds the output of the multiplier 124 andthe inverted output of the adder 129, effecting subtraction. The adder133 adds the output of the multiplier 123 and the output of the adder130. The adder 134 adds the output of the multiplier 123 and theinverted output of the adder 130, thereby performing subtraction.

The output of the adder 133 is a coefficient f(0) that has beengenerated by fourth-order inverse discrete cosine transform. Similarly,the output of the adder 131 is a coefficient f(1), the output of theadder 132 is a coefficient f(2), and the output of the adder 134 is acoefficient f(3).

That is, nine adders and five multipliers cooperate in the section 105and accomplish a 4×4 inverse discrete-cosine transform. The valueC_(3/8) is given as follows:C_(3/8)=cos (3δ/8)

The matrix representing the compression inverse discrete-cosinetransform (field separation) performed in the section 106 can bedecomposed by using the Wang algorithm, as shown in the followingequation (3). In the equation (3), A to J at the multipliers are of thesame meaning as in the equation (1). $\begin{matrix}{{{Equation}\quad 6{\text{:}\left\lbrack {FS}^{I} \right\rbrack}} = {\begin{bmatrix}1 & 0 & 0 & 0 \\0 & 0 & 0 & 1 \\0 & 1 & 0 & 0 \\0 & 0 & 1 & 0\end{bmatrix} \cdot {\quad{{\begin{bmatrix}1 & 0 & 1 & 0 \\0 & 1 & 0 & 1 \\1 & 0 & {- 1} & 0 \\0 & 1 & 0 & {- 1}\end{bmatrix} \cdot {\begin{bmatrix}\left\lbrack M_{1} \right\rbrack & \quad \\\quad & \left\lbrack M_{2} \right\rbrack\end{bmatrix}\begin{bmatrix}10000000 \\00100000 \\00001000 \\00000010 \\00010000 \\00000100 \\01000000 \\00000001\end{bmatrix}}}\quad\cdots\quad(3)}}}} & \quad\end{matrix}$

The matrices [M₁] and [M₂] in the equation (3) are expressed as follows:$\begin{matrix}{{{Equation}{\quad\quad}7{\text{:}\quad\left\lbrack M_{1} \right\rbrack}} = {\begin{bmatrix}1 & 1 \\1 & {- 1}\end{bmatrix} \cdot \begin{bmatrix}1 & 0 & 0 & 0 \\0 & 1 & 1 & 1\end{bmatrix} \cdot \begin{bmatrix}{A\quad 000} \\{0\quad D\quad 00} \\{00\quad F\quad 0} \\{000\quad H}\end{bmatrix}}} & \quad \\{{{Equation}\quad 8{\text{:}\quad\left\lbrack M_{1} \right\rbrack}} = {\begin{bmatrix}110 \\101\end{bmatrix} \cdot \begin{bmatrix}{- 1} & 1 & 0 & 0 & 0 & 0 \\0 & 0 & 1 & 0 & 1 & 0 \\0 & 0 & 0 & 1 & 0 & 1\end{bmatrix} \cdot \begin{bmatrix}{E\quad 000} \\{0\quad G\quad 00} \\{00\quad B\quad 0} \\{00\quad C\quad 0} \\{000\quad I} \\{000\quad J}\end{bmatrix}}} & \quad\end{matrix}$

FIG. 7 illustrates how the compression inverse discrete-cosine transform(field separation) section 106 carries out compression inversediscrete-cosine transform of field separation type, by using the Wangalgorithm.

As shown in FIG. 7, a multiplier 141 multiplies a coefficient F(0),i.e., one of eighth-order coefficients F(0) to F(7), by the coefficientA described in the equation (1). A multiplier 142 multiplies acoefficient F(2) by the coefficient D also described in the equation(1). A multiplier 143 multiplies a coefficient F(4) by the coefficient Fdescribed in the equation (1). A multiplier 144 multiplies a coefficientF(6) by the coefficient H described in the equation (1). A multiplier145 multiplies a coefficient F(3) by the coefficient E described in theequation (1).

A multiplier 146 multiplies a coefficient F(S) by the coefficient Gdescribed in the equation (1). A multiplier 147 multiplies a coefficientF(1) by the coefficient B described in the equation (1). A multiplier148 multiplies a coefficient F(1) by the coefficient C described in theequation (1). A multiplier 149 multiplies a coefficient F(7) by thecoefficient I described in the equation (1). A multiplier 150 multipliesa coefficient F(7) by the coefficient J described in the equation (1).

The output of the multiplier 141 is input to adders 156 and 157. Theoutput of the multiplier 142 is input to an adder 151. The output of themultiplier 143 is input to the adder 151, too. The output of themultiplier 143 is input to the adder 151. The output of the multiplier144 is input to an adder 152. The output of the multiplier 145 is inputto an adder 153. The output of the multiplier 146 is inverted and theninput to the adder 153. The output of the multiplier 147 is input to anadder 154. The output of the multiplier 148 is input to an adder 155.The output of the multiplier 149 is input to the adder 154. The outputof the multiplier 150 is input to the adder 155.

The adder 151 adds the outputs of the multipliers 142 and 143. The adder152 adds the output of the adder 151 and the output of the multiplier144. The adder 153 adds the output of the multiplier 145 and theinverted output of the multiplier 146, thus carrying out subtraction.The adder 154 adds the outputs of the multipliers 147 and 149. The adder155 adds the outputs of the adders 148 and 150.

The adder 156 adds the output of the multiplier 141 and the output ofthe adder 152. The adder 157 adds the output of the multiplier 141 andthe output of the adder 152. An adder 158 adds the outputs of the adders153 and 154. An adder 159 adds the outputs of the adders 153 and 155.

An adder 160 adds the outputs of the adders 156 and 158. An adder 161adds the outputs of the adders 156 and 158. An adder 162 adds theoutputs of the adders 157 and 159. An adder 163 adds the outputs of theadders 157 and 159.

The output of the adder 160 is a coefficient f(0) that has beensubjected to inverse discrete cosine transform of field separation type.The output of the adder 162 is a similar coefficient f(2), the output ofthe adder 161 is a similar coefficient f(3), and the output of the adder163 is a similar coefficient f(1).

As has been described with reference to FIG. 7, the section 106accomplishes compression inverse discrete-cosine transform of fieldseparation type, by using thirteen adders and ten multipliers.

Motion-compensating devices that operate in a field-motion compensationmode and a frame-motion compensation mode, respectively, will bedescribed. In both compensation modes, a twofold interpolation filter,such as a half-band filter, generates a ½-precision pixel and a¼-precision pixel is generated from the ½-precision pixel by means oflinear interpolation, thus achieving interpolation in horizontaldirection. In this process, a half-band filter may be used to output, asa predicted value, a pixel that has the same phase as the pixel readfrom a frame memory. If this is the case, it is unnecessary to repeatmultiplication and addition as many times as the number of taps that areprovided. This helps achieve a high-speed operation. The use of thehalf-band filter renders it possible to accomplish the division in theprocess of interpolation, by means of shift operation. The speed ofoperation can therefore increase further. Alternatively, filtering offourfold interpolation may be performed, thereby to generate pixels thatare required to achieve motion compensation.

The operating principles of the motion-compensating (field prediction)sections 108 and 109 will be explained below.

In the section 108 (field prediction) and the section 109 (frameprediction), a twofold interpolation filter, such as a half-band filter,generates a ½-precision pixel and a ¼-precision pixel is generated fromthe ½-precision pixel by means of linear interpolation, thus achievinginterpolation in horizontal direction. In this case, a half-band filtermay be used to output, as a predicted value, a pixel that has the samephase as the pixel read from the frame memory 110. If so, multiplicationor addition need not be repeated as many times as the number of tapsthat are provided. This helps achieve a high-speed operation. The use ofthe half-band filter makes it possible to accomplish the division in theprocess of interpolation, by means of shift operation. The speed ofoperation can therefore increase further. Alternatively, filtering offourfold interpolation may be performed, thereby to generate pixels thatare required to achieve motion compensation.

FIGS. 8A, 8B and 8C are diagrams which explain how themotion-compensating sections (field prediction) 108 performsinterpolation in vertical direction. First, as shown in FIG. 8A, thesection 108 reads pixel values ga that have a phase difference betweenfields, from the video memory 110, in accordance with the vector valuecontained in the input compressed image data (bit stream). Then, asshown in FIG. 8B, a twofold interpolation filter generates a ½-precisionpixel gb in each field. As shown in FIG. 8C, the motion-compensatingsections (field prediction) 108 carries out intra-field linearinterpolation, thereby generating a ¼-precision pixel gc. A half-bandfilter may be used as the twofold interpolation filter and may outputpixel values of the same phase as those read from video memory 110,which represent a predicted image. In this case, multiplication oraddition need not be repeated as many times as the number of taps thatare provided. This achieves a high-speed operation. Alternatively,filtering of fourfold interpolation may be effected, thereby to generatepixel values similar to the ones shown in FIG. 8C, from pixel values gaillustrated in FIG. 8A

FIGS. 9A, 9B and 9C are diagrams which explain how themotion-compensating sections (frame prediction) 109 performsinterpolation in vertical direction. First, as shown in FIG. 9A, thesection 109 reads pixel values ga that have a phase difference betweenfields, from the video memory 110, in accordance with the vector valuecontained in the input compressed image data (bit stream). Then, asshown in FIG. 9B, a twofold interpolation filter generates a ½-precisionpixel gb in each field. As shown in FIG. 9C, the motion-compensatingsections (frame prediction) 109 performs inter-field linearinterpolation, thereby generating a ¼-precision pixel gc. Theinterpolation prevents inversion of fields and

mixing of fields that may deteriorate image quality. A half-band filtermay be is used as the twofold interpolation filter and may output pixelvalues of the same phase as those read from video memory 110, whichrepresent a predicted image. In this case, multiplication or additionneed not be repeated as many times as the number of taps that areprovided. This achieves a high-speed operation.

In practice, coefficients are prepared and applied, so that the two-stepinterpolation, which consists in using a twofold interpolation filterand performing linear interpolation, may be accomplished in a singlestep in both the horizontal direction and the vertical direction. Onlynecessary pixel values are generated in both the horizontalinterpolation and the vertical interpolation, in accordance with themotion vectors contained in the input compressed image data (bitstream). Filter coefficients that correspond to the horizontal andvertical motion vectors may be prepared. In this case, the horizontalinterpolation and the vertical interpolation can be carried out at atime.

To effect twofold interpolation filtering, it may be necessary,depending on the values of motion vectors, to refer to the data storedin video memory 110 and existing outside the image frame. If so, a rowof pixels is folded at the end point, for the number of taps required,as is illustrated in FIG. 10A. This process shall be called “mirrorprocess.” Alternatively, pixels identical in value to the pixel at theend point may be provided outside the image frame in the same number asthe taps required, as is illustrated in FIG. 10B. The alternativeprocess shall be called “hold process.” Either the mirror process or thehold process is carried out in units of fields to achieve verticalinterpolation in both the motion-compensating sections (fieldprediction) 108 and the motion-compensating sections (frame prediction)109.

The operating principle of the frame-converting section 111 will bedescribed below.

If the image represented by the compressed image data (bit stream) inputmay consists of 1920×1080 pixels, the image data output from the videomemory 110 is composed of 960×1080 pixels. In order to output the imagedata to, for example, a display apparatus for displaying a 720×480 pixelimage (aspect ratio of 16:9), pixel data items must be extracted toreduce the number of pixels to ¾ in the horizontal direction and to 4/9in the vertical direction. The frame-converting section 111 extractspixel data items in this manner, thereby to change the size of the imageframe.

The MPEG2 data-encoding system described above can process high-qualityimage codes that are suitable for broadcasting, but cannot process asmaller amount (bit rate) of image data than MPEG1 image data. In otherwords, the MPEG2 data-encoding system cannot process image codes thatare compressed at higher ratios. More and more image codes compressed athigher ratios will be used because mobile telephones are now used inincreasing numbers. To cope with this situation, the MPEG4 data-encodingsystem has been standardized. The standards of the MPEG4 data-encodingsystem was approved internationally in December 1998, as ISO/IEC14496-2.

It is demanded that the MPEG2-image compressed data (bit stream) encodefor use in the digital broadcasting be converted to MPEG4-imagecompressed data (bit stream) of a smaller amount (bit rate) that can bereadily processed in mobile terminals and the like.

To meet this demand, an image data converting apparatus (known as“transcoder”) has been proposed. FIG. 11 illustrates such an apparatusdescribed in “Field-to-Frame Transcoding with Spatial and TemporalDownsampling” (Susie J. Wee, John G. Apostolopoulos, and Nick Feamster,ICIP '99).

As shown in FIG. 11, the frame data items contained in MPEG2-imagecompressed data (bit stream), i.e., interlaced-scan image data, is inputto the picture-type determining section 112.

The picture-type determining section 112 determines whether the eachframe data item pertains to an I/P (intra-image encoded image/forwardprediction encoded image) or a B picture (bi-directional predictionencoded image). If the frame data item pertains to an I/P picture, thesection 112 outputs the data about the I/P picture to the MPEG2-imagedecoding section (I/P picture) 113.

The MPEG2-image decoding section (I/P picture) 113 performs the sameprocess as does the ordinary MPEG2-image decoding apparatus. However,the section 113 needs to decode the I/P picture only. This is becausethe picture-type determining section 112 has discarded the data aboutthe B picture. The pixel value, i.e., the output of the picture-typedetermining section 112, is input to the data-extracting section 114.

The data-extracting section 114 carries out ½ extraction in thehorizontal direction. In the vertical direction, the section 114extracts the first field or the second field, preserving one of thesetwo fields, thereby generating sequential-scan image data. Thesequential-scan image data has a ¼ size with respect to the image datainput to the picture-type determining section 112. The sequential-scanimage data generated by the data-extracting section 114 is input to theMPEG4-image encoding section (I/P-VOP) 115.

The MPEG4-image encoding section (I/P-VOP) 115 encodes the signalsconstituting the sequential-scan image data input to it, thus generatingMPEG4-image compressed data (bit stream).

The MPEG2-image compressed data (bit stream) is supplied from theMPEG2-image decoding section 113 to the motion-vector synthesizingsection 116. The section 116 performs mapping on the motion vectorcontained in the MPEG2-image compressed data, thereby synthesizing amotion vector for the image data some part of which has been extractedby the data-extracting section 114. The motion vector, thus generated,is supplied to the motion-vector detecting section 117. The section 117detects a high-precision motion vector from the motion vectorsynthesized by the motion-vector synthesizing section 116.

In MPEG4-image data, VOP (Video Object Plane) represents a region thatis composed of one or more macro blocks that surround an object. The VOPregion is classified into any one of an I picture, a P picture and a Bpicture, depending on the encoding scheme employed. An I-VOP (i.e., VOPof I picture), or an image (region), is encoded (by means ofintra-encoding), without being subjected to motion compensation. A P-VOP(i.e., VOP of P picture) is encoded by means of forward predictionencoding, on the basis of the image (either an I picture or a P-VOP)that precedes in time. A B-VOP (i.e., VOP of B picture) is encoded bymeans of bi-directional prediction encoding, on the basis of two images(I pictures or P-VOPs) that follows in time.

Thus, the image data converting apparatus shown in FIG. 11 can convertMPEG2-image compressed data (bit stream) to MPEG4-image compressed data(bit stream).

In the image data converting apparatus of FIG. 11, however, theMPEG2-image decoding section (I/P picture) 113 must process a greatamount of data. The video memory 110 should therefore have a largestorage capacity. Inevitably, it is difficult to provide an inexpensiveapparatus if only dedicated LSIs are used. The apparatus may comprisegeneral-purpose processors. In this case, the apparatus may fail tooperate in real time.

BRIEF SUMMARY OF THE INVENTION

The present invention has been made in consideration of the foregoing.The object of the invention is to provide an apparatus and method thatcan convert, at low cost and in real time, interlaced-scan MPEG2-imagecompressed data (bit stream) to sequential-scan MPEG4-image compresseddata (bit stream).

An image data converting apparatus according to the invention isdesigned to convert first compressed image data to second compressedimage data more compressed than the first compressed image data. Thefirst compressed image data is interlaced-scan data that has beencompressed by orthogonal transform and motion compensation, and thesecond compressed data is serial-scan data. The apparatus comprises:image data decoding means for decoding the first compressed image databy using only lower mth-order orthogonal transform coefficients includedin nth-order orthogonal transform coefficients (where m<n), in both avertical direction and a horizontal direction in the first compressedimage data; scan-converting means for converting interlaced-scan dataoutput from the image data decoding means to serial-scan data; and thesecond image data encoding means for encoding the serial-scan data,thereby generating the second compressed image data.

In the image data converting apparatus, the first compressed image datais MPEG2-image compressed data containing eighth-order discrete cosinetransform coefficients in both the vertical direction and the horizontaldirection. The image data decoding means is MPEG2-image data decodingmeans for decoding the MPEG2-image compressed data in both the verticaldirection and the horizontal direction, by using only lower fourth-ordercoefficients included in eighth-order discrete cosine transformcoefficients. The image data encoding means is MPEG4-image encodingmeans for encoding the serial-scan data, thereby generating MPEG4-imagecompressed data.

An image data converting method according to the invention is designedto convert first compressed image data to second compressed image datamore compressed than the first compressed image data. The firstcompressed image data is interlaced-scan data that has been compressedby orthogonal transform and motion compensation. The second compresseddata is serial-scan data. The method comprises the steps of: decodingthe first compressed image data by using only lower mth-order orthogonaltransform coefficients included in nth-order orthogonal transformcoefficients (where m<n), in both a vertical direction and a horizontaldirection in the first compressed image data; converting interlaced-scandata output from the image data decoding means to serial-scan data; andencoding the serial-scan data, thereby generating the second compressedimage data.

In the image data converting method, the first compressed image data isMPEG2-image compressed data containing eighth-order discrete cosinetransform coefficients in both the vertical direction and the horizontaldirection. The step of the decoding the first compressed image data isto decode the MPEG2-image compressed data in both the vertical directionand the horizontal direction, by using only lower fourth-ordercoefficients included in eighth-order discrete cosine transformcoefficients. The step of encoding the serial-scan data is to encode theserial-scan data, thereby generating MPEG4-image compressed data.

In the image converting apparatus and method according to the invention,the picture type of each frame data contained in the interlaced-scanMPEG4-picture compressed data is determined. In accordance with thepicture type determined, only the frame data about an intra-imageencoded image/forward prediction encoded image is output, whilediscarding the frame data about a bi-directional prediction encodedimage, thereby converting the frame rate.

In other words, outputs only the frame data about I/P pictures,contained in the input MPEG4-picture compressed data (bit stream), isoutput, while the frame data about B pictures, thereby converting theframe rate. Part of the data about the I/P pictures is then decoded inboth the horizontal direction and the vertical direction, by using onlythe fourth-order coefficients included in the eighth-order DCTcoefficients. Thereafter, of the pixel values to be output as decodedMPEG2-image compressed data, only the data of the first field or thesecond field is preserved, and the data of the other of these two fieldsis discarded. Further, twofold interpolation is performed, therebyconverting the data to serial-scan data. The serial-scan data isencoded, generating MPEG4-image compressed data (bit stream). Moreover,mapping is performed on the motion vector contained in the serial-scanimage data, on the basis of the motion vector contained in the inputcompressed image data (bit stream). Furthermore, a high-precision motionvector is detected from the motion vector subjected to the mapping.

According to the present invention, the first compressed image data isdecoded by using only lower mth-order orthogonal transform coefficientsincluded in nth-order orthogonal transform coefficients (where m<n), inboth a vertical direction and a horizontal direction in the firstcompressed image data. Then, interlaced-scan data is converted toserial-scan data. Further, the second compressed image data is generatedfrom the serial-scan data. Hence, interlaced-scan MPEG2-image compresseddata (bit stream) can be converted to sequential-scan MPEG4-imagecompressed data (bit stream) at low cost and in real time, by processinga small amount of data and using a video memory of small storagecapacity.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a block diagram showing an image data decoding apparatus(i.e., 4×4 down decoder) that decodes data by using only fourth-orderlower data in both the vertical direction and the horizontal direction;

FIGS. 2A and 2B are diagrams explaining the two operating principles thevariable-length decoding section provided in the apparatus of FIG. 1assumes to decode MPEG2-image compressed data generated by zigzagscanning;

FIGS. 3A and 3B are diagrams explaining two operating principles thevariable-length decoding section provided in the apparatus of FIG. 1assumes to decode MPEG2-image compressed data generated by alternatescanning;

FIGS. 4A and 4B are diagrams explaining the phases of the pixel valuesstored in the video memory that is incorporated in the apparatus of FIG.1;

FIG. 5 is a diagram explaining the operating principle of thecompression inverse discrete-cosine transform section (field separation)provided in the apparatus of FIG. 1;

FIG. 6 is a diagram illustrating how the compression inversediscrete-cosine transform (4×4) section used in the apparatus of FIG. 1performs its function by using a fast algorithm;

FIG. 7 is a diagram illustrating how the compression inversediscrete-cosine transform (field separation) section provided in theapparatus of FIG. 1 performs its function by using a fast algorithm;

FIGS. 8A, 8B and 8C are diagrams explaining the operating principle ofthe motion-compensating section (field prediction) that is incorporatedin the apparatus of FIG. 1;

FIGS. 9A, 9B and 9C are diagrams explaining the operation principle ofthe motion-compensating section (frame prediction) that is provided inthe apparatus of FIG. 1;

FIGS. 10A and 10B are diagrams explaining the mirror process and holdprocess that are performed by the motion-compensating section (fieldprediction) and motion-compensating section (frame prediction), bothprovided in the apparatus of FIG. 1;

FIG. 11 is a block diagram showing an image data converting apparatus(transcoder) that receives MPEG2-image compressed data (bit stream) andoutputs MPEG4-image compressed data (bit stream);

FIG. 12 is a block diagram schematically showing an image dataconverting apparatus that is an embodiment of the present invention;

FIG. 13 is a diagram explaining how the amount of data processed isdecreased in the apparatus of FIG. 12 when the input compressed imagedata (bit stream) contains a macro block of frame DCT mode; and

FIGS. 14A, 14B and 14C are diagrams explaining the operating principleof the scan-converting section incorporated in the image data convertingapparatus of FIG. 12.

DETAILED DESCRIPTION OF THE INVENTION

A preferred embodiment of the present invention will be described, withreference to the accompanying drawings.

FIG. 12 is a schematic representation of an image data convertingapparatus according to the invention.

As shown in FIG. 12, the frame data items contained in MPEG2-imagecompressed data (bit stream), i.e., interlaced-scan image data, is inputthe picture-type determining section 18.

The picture-type determining section 18 supplies the data about an I/Ppicture to the MPEG2-image data decoding section (i.e., I/P picture 4×4down-decoder) 19, but discards the data about a B picture. Thus, thesection 18 converts the frame rate.

The MPEG2-image data decoding section 19 performs the same process asdoes the apparatus shown in FIG. 1. However, the section 19 only needsto decode the I/P picture. This is because the picture-type determiningsection 18 already discarded the data about the B picture. TheMPEG2-image data decoding section 19 only needs to decode data by usingonly fourth-order lower data (DCT coefficients) in both the verticaldirection and the horizontal direction, as in the image data decodingapparatus shown in FIG. 1. Hence, it suffices for the video memory forstoring the data the section 19 processes to have only one fourth of thestorage capacity required in the image data converting apparatus of FIG.11, which comprises the MPEG2-image decoding section (I/P picture) 113.Further, the amount of data processed to achieve inverse discrete cosinetransform decreases to a quarter (¼) in the field DCT mode, and to half(½) in the frame DCT mode. Moreover, in the frame DCI mode, some of the(4×8)th-order discrete cosine transform coefficients may be replaced byOs as shown in FIG. 13. If this case, the amount of data to be processedcan be reduced, causing virtually no deterioration of image quality.

The output of the MPEG2-image data decoding section (i.e., I/P picture4×4 down-decoder) 19, or interlaced-scan pixel data, is input to thescan-converting section 20. The scan-converting section 20 preserves oneof two fields and discards the other of the two fields. The section 20then performs twofold interpolation in the field preserved, therebyconverting the interlaced-scan pixel data to sequential-scan pixel dataas is illustrated in FIGS. 14A, 14B and 14C. More specifically, thesection 20 preserves only the pixels ga of the first field arepreserved, while discarding the pixel values of the second field, as isshown in FIGS. 14A and 14B. Next, the scan-converting section 20 carriesout twofold interpolation in the first field (i.e., preserved field) asillustrated in FIG. 14C, thereby generating interpolated pixels gc.Thus, the scan-converting section 20 converts the interlaced-scan pixeldata to sequential-scan pixel data. The section 19 outputs thesequential-scan pixel data.

The sequential-scan pixel data is supplied to the MPEG4-image encodingsection (I/P-VOP) 21. The MPEG4-image encoding section (I/P-VOP) 21encodes the signals constituting the sequential-scan pixel data input toit, thus generating MPEG4-image compressed data (bit stream).

The motion vector data contained in the MPEG2-image compressed data (bitstream) input, which has been detected by the MPEG2-image data decodingsection (i.e., I/P picture 4×4 down-decoder), is input to themotion-vector synthesizing section 22. The section 22 performs mappingon the motion vector data, thereby synthesizing a motion vector valuefor the converted serial-scan image. The motion vector value is suppliedto the motion-vector detecting section 23. The section 23 detects ahigh-precision motion vector from the motion vector value in theconverted serial-scan image.

As has been described, the image data converting apparatus according tothe present invention decodes the interlaced-scan MPEG2-image compresseddata by using only fourth-order lower data in both the verticaldirection and the horizontal direction. The apparatus then converts theinterlaced-scan data to serial-scan data, thereby generating MPEG4-imagecompressed data. Thus, the apparatus can convert, at low cost and inreal time, interlaced-scan MPEG2-image compressed data (bit stream) tosequential-scan MPEG4-image compressed data (bit stream).

As described above, the data input to the image data convertingapparatus of the invention is MPEG2-image compressed data (bit stream),and the data output from the apparatus is MPEG4-image compressed data(bit stream). The input and output are not limited to these,nonetheless. For example, the input may be MPEG1-image compressed data(bit stream) or H.263 compressed image data (bit stream). The apparatusaccording to this invention can converts these types of compressed data,too, at low cost and in real time.

1. An image data converting apparatus for converting first compressedimage data to second compressed image data being more compressed thanthe first compressed image data, said first compressed image data beinginterlaced-scan data compressed by orthogonal transform and motioncompensation, and said second compressed data being serial-scan data,said apparatus comprising: image data decoding means for decoding thefirst compressed image data by using only lower mth-order orthogonaltransform coefficients included in nth-order orthogonal transformcoefficients (where m<n), in both a vertical direction and a horizontaldirection in the first compressed image data; scan-converting means forconverting interlaced-scan data output from the image data decodingmeans to serial-scan data; and image data encoding means for encodingthe serial-scan data, thereby generating the second compressed imagedata.
 2. The apparatus according to claim 1, wherein the firstcompressed image data is MPEG2-image compressed data containingeighth-order discrete cosine transform coefficients in both the verticaldirection and the horizontal direction, the image data decoding means isMPEG2-image data decoding means for decoding the MPEG2-image compresseddata in both the vertical direction and the horizontal direction, byusing only lower fourth-order coefficients included in the eighth-orderdiscrete cosine transform coefficients, and the image data encodingmeans is MPEG4-image encoding means for encoding the serial-scan datafrom the scan converting means, thereby generating MPEG4-imagecompressed data.
 3. The apparatus according to claim 2, furthercomprising picture-type determining means for determining a code type ofeach frame in the interlaced-scan MPEG2-image compressed data, foroutputting data about an intra-image encoded image/forward predictionencoded images and for discarding data about a bi-directional predictionencoded image, thereby to convert a frame rate, wherein an output of thepicture-type determining means is input to the MPEG2-image data decodingmeans.
 4. The apparatus according to claim 3, wherein the MPEG2-imagedata decoding means decodes only the intra-image encoded image/forwardprediction encoded image.
 5. The apparatus according to claim 2, whereinthe MPEG2-image data decoding means comprises variable-length decodingmeans, and the variable-length decoding means performs variable-lengthencoding on only discrete cosine transform coefficients required in adiscrete cosine transform, in accordance with whether a macro block ofthe input MPEG2-image compressed data is of a field-discrete cosinetransform mode or a fame-discrete cosine transform mode.
 6. Theapparatus according to claim 2, wherein the MPEG2-image data decodingmeans comprises compression inverse discrete-cosine transform means of afield-discrete cosine transform mode, the compression inversediscrete-cosine transform means extracts only the lower fourth-ordercoefficients included in the eighth-order discrete cosine transformcoefficients, in both the vertical direction and the horizontaldirection, and then performs a fourth-order inverse discrete cosinetransform on the lower fourth-order coefficients extracted.
 7. Theapparatus according to claim 6, wherein the inverse discrete-cosinetransform is carried out in both the horizontal direction and thevertical direction by a method based on a predetermined fast algorithm.8. The apparatus according to claim 2, wherein the MPEG2-image datadecoding means comprises compression inverse discrete-cosine transformmeans of a frame-discrete cosine transform mode, wherein the compressioninverse discrete-cosine transform means extracts only the lowerfourth-order coefficients included in the eighth-order discrete cosinetransform coefficients in the horizontal direction, performsfourth-order inverse discrete cosine transform on the lower fourth-ordercoefficients extracted, and performs a field-discrete cosine transformin the vertical direction.
 9. The apparatus according to claim 8,wherein the inverse discrete-cosine transform is carried out in both thehorizontal direction and the vertical direction by a method based on apredetermined fast algorithm.
 10. The apparatus according to claim 8,wherein the compression inverse discrete-cosine transform means offrame-discrete cosine transform mode performs the inverse discretecosine transform by using only (4×4+4×2)th-order coefficients includedin (4×8)th-order discrete cosine transform coefficients input to achievethe field-discrete compression inverse discrete cosine transform, whilereplacing remaining coefficients by Os, thus discarding the remainingcoefficients.
 11. The apparatus according to claim 2, wherein theMPEG2-image data decoding means comprises motion-compensating means,wherein the motion-compensating means performs ¼-precision pixelinterpolation in both the horizontal direction and the verticaldirection in accordance with a motion vector contained in the inputMPEG2-image compressed data.
 12. The apparatus according to claim 11,wherein the motion-compensating means initially performs ½-precisionpixel interpolation in the horizontal direction by using a twofoldinterpolation digital filter and then performs the ¼-precision pixelinterpolation by means of linear interpolation.
 13. The apparatusaccording to claim 11, wherein the motion-compensating means initiallyperforms ½-precision pixel interpolation in a field, as verticalinterpolation by using a twofold interpolation digital filter, and thenperforms the ¼-precision pixel interpolation in the field by means oflinear interpolation, when a macro block of the input MPEG2-imagecompressed data is of a field prediction mode.
 14. The apparatusaccording to claim 11, wherein the motion-compensating means initiallyperforms ½-precision pixel interpolation in a field, as verticalinterpolation by using a twofold interpolation digital filter, and thenperforms the ¼-precision pixel interpolation in the field by means oflinear interpolation, when a macro block of the input MPEG2-imagecompressed data is of a frame prediction mode.
 15. The apparatusaccording to claim 11, wherein the motion-compensating means includes ahalf-band digital filter for performing the pixel interpolation in boththe horizontal direction and the vertical direction.
 16. The apparatusaccording to claim 11, wherein the MPEG2-image data decoding meansfurther comprises storage means for storing pixel values, and themotion-compensating means calculates coefficients equivalent to asequence interpolating operation and applies the coefficients, therebyto perform motion compensation on the pixel values read from the storagemeans in accordance with the motion vector contained in the inputMPEG2-image compressed data.
 17. The apparatus according to claim 11,wherein, when pixel values outside an image frame are required toachieve twofold interpolation, the motion-compensating means performsone of a mirror process and a hold process, thereby generating a numberof virtual pixel values as the equal to a number of taps provided in afilter in order to accomplish motion compensation, before performing themotion compensation.
 18. The apparatus according to claim 17, whereinthe motion-compensating means performs one of the mirror process er andthe hold process in units of fields.
 19. The apparatus according toclaim 2, wherein the scan-converting means preserves one of a firstfield and a second field of the interlaced-scan image data output fromthe MPEG2-image data decoding means, discards the one of the first andsecond fields not preserved, and performs twofold up-sampling onpreserved pixel values, thereby converting the interlaced-scan data toserial-scan data.
 20. The apparatus according to claim 2, wherein theMPEG2-image data decoding means has the function of encoding only aregion composed of one or more macro blocks that surround an object inan intra-image encoded image/forward prediction encoded image.
 21. Theapparatus according to claim 2, further comprising motion-vectorsynthesizing means for generating a motion vector value corresponding tothe image data subjected to scan conversion, from a motion vector datacontained in the input MPEG2-image compressed data.
 22. The apparatusaccording to claim 21, further comprising motion-vector detecting meansfor detecting a high-precision motion vector from the motion vectorvalue generated by the motion-vector synthesizing means.
 23. An imagedata converting method of converting first compressed image data tosecond compressed image data being more compressed than the firstcompressed image data, said first compressed image data beinginterlaced-scan data compressed by orthogonal transform and motioncompensation, and said second compressed data being serial-scan data,said method comprising the steps of: decoding the first compressed imagedata by using only lower mth-order orthogonal transform coefficientsincluded in nth-order orthogonal transform coefficients (where m<n), inboth a vertical direction and a horizontal direction in the firstcompressed image data; converting interlaced-scan data output from thestep of decoding to serial-scan data; and encoding the serial-scan data,thereby generating the second compressed image data.
 24. The methodaccording to claim 23, wherein the first compressed image data isMPEG2-image compressed data containing eighth-order discrete cosinetransform coefficients in both the vertical direction and the horizontaldirection, the step of decoding the first compressed image data decodesthe MPEG2-image compressed data in both the vertical direction and thehorizontal direction, by using only lower fourth-order coefficientsincluded in the eighth-order discrete cosine transform coefficients, andthe step of encoding the serial-scan data encodes the serial-scan data,thereby generating MPEG4-image compressed data.
 25. The method accordingto claim 24, wherein the code type of each frame in the interlaced-scanMPEG2-image compressed data is determined, data about an intra-imageencoded image/forward prediction encoded image is output in accordancewith the code type determined, data about a bi-directional predictionencoded image is discarded thereby to convert a frame rate, and theMPEG4-image compressed data is generated from the converted frame rate.26. The method according to claim 25, wherein only the intra-imageencoded image/forward prediction encoded image is decoded in the step ofdecoding the MPEG2-image compressed data.
 27. The method according toclaim 24, wherein in the step of decoding the MPEG2-image compresseddata, variable-length decoding is performed on only the discrete cosinetransform coefficients required in a discrete cosine transform, inaccordance with whether a macro block of the input MPEG2-imagecompressed data is one of a field-discrete cosine transform mode and aframe-discrete cosine transform mode.
 28. The method according to clam24, wherein in the step of decoding the MPEG2-image compressed data, aninverse discrete-cosine transform of a field-discrete cosine transformmode is performed by extracting only the lower fourth-order coefficientsincluded in the eighth-order discrete cosine transform coefficients, inboth the vertical direction and the horizontal direction, and then byperforming fourth-order inverse discrete cosine transform on theextracted lower fourth-order coefficients.
 29. The method according toclaim 28, wherein the inverse cosine transform is carried out in boththe horizontal direction and the vertical direction, by a method basedon a predetermined fast algorithm.
 30. The method according to claim 24,wherein in the step of decoding the MPEG2-image compressed data, acompression inverse discrete-cosine transform of a frame-discrete cosinetransform mode is performed by extracting only the lower fourth-ordercoefficients included in eighth-order discrete cosine transformcoefficients and then fourth-order inverse discrete cosine transform isperformed on the extracted lower fourth-order coefficients, in thehorizontal direction, and field-discrete cosine transform is performedin the vertical direction.
 31. The method according to claim 30, whereinthe inverse cosine transform is carried out in both the horizontaldirection and the vertical direction, by a method based on apredetermined fast algorithm.
 32. The method according to claim 30,wherein in the compression inverse discrete-cosine transform offrame-discrete cosine transform mode, only (4 4+4 2)th-ordercoefficients included in (4 8)th-order discrete cosine transformcoefficients input are used to achieve inverse cosine transform, whilereplacing the remaining coefficients by 0s.
 33. The method according toclaim 24, wherein in motion compensation performed in the step ofdecoding the MPEG2-image compressed data, ¼-precision pixelinterpolation is carried out in both the horizontal direction and thevertical direction, in accordance with a motion vector contained in theinput MPEG2-image compressed data.
 34. The method according to claim 33,wherein in the step of performing motion compensation, ½-precision pixelinterpolation is initially performed in the horizontal direction byusing a twofold interpolation digital filter and then ¼-precision pixelinterpolation is performed by means of linear interpolation.
 35. Themethod according to claim 33, wherein in the step of performing motioncompensation, ½-precision pixel interpolation is initially performed ina field, as vertical interpolation, by using a twofold interpolationdigital filter, and then ¼-precision pixel interpolation is performed inthe field by means of linear interpolation, when a macro block of theinput MPEG2-image compressed data is of a field prediction mode.
 36. Themethod according to claim 33, wherein in the step of performing motioncompensation, ½-precision pixel interpolation is initially performed ina field, as vertical interpolation, by using a twofold interpolationdigital filter, and then the ¼-precision pixel interpolation isperformed in the field by means of linear interpolation, when a macroblock of the input MPEG2-image compressed data is of a frame predictionmode.
 37. The method according to claim 36, wherein in the step ofperforming motion compensation, a half-band filter is used as thetwofold interpolation digital filter, to perform the interpolation. 38.The method according to claim 33, wherein in the step of decoding theMPEG2-image compressed data, pixel values are stored, and in the step ofperforming motion compensation, coefficients already calculated andequivalent to a sequence interpolating operations are applied, therebyto perform motion compensation on the stored pixel values, in accordancewith the motion vector contained in the input MPEG2-image compresseddata.
 39. The method according to claim 33, wherein, when pixel valuesoutside an image frame are required to achieve twofold interpolation,one of mirror process and a hold process is performed, therebygenerating a number of virtual pixel values equal to a number of tapsprovided in a filter required in order to accomplish the motioncompensation.
 40. The method according to claim 39, in the step ofperforming the motion compensation, the mirror process or the holdprocess is carried out in units of fields.
 41. The method according toclaim 24, wherein in the step of converting, a first field or a secondfield of the interlaced-scan image data is preserved, and the one of thefirst and second fields that is not preserved is discarded, and twofoldup-sampling is performed on preserved pixel values, thereby convertingthe interlaced-scan data to serial-scan data, said first and secondfields being contained in the MPEG2-image compressed data that has beendecoded.
 42. The method according to claim 24, wherein only a regioncomposed of one or more macro blocks that surround an object in anintra-image encoded image/forward prediction encoded image is encoded inthe step of decoding the MPEG2-image compressed data.
 43. The methodaccording to claim 24, wherein a motion vector value corresponding tothe image data subjected to scan conversion is synthesized from motionvector data contained in the input MPEG2-image compressed data.
 44. Themethod according to claim 43, wherein a high-precision motion vector isdetected from the motion vector value that has been synthesized.